1. Field of the Invention
The present invention relates to the field of particle detector instrumentation. More particularly, the present invention relates to electron multipliers wherein real time signal generated by an intermediate dynode is monitored to regulate in real time the gain to dynodes near the output of the instrument.
2. Discussion of the Related Art
Electron multipliers are often utilized as detectors for the detection of particles such as photons, neutral molecules, and/or ions provided by mass spectrometry. While the geometry of such devices can vary, a common beneficial design comprises a cathode, an anode, and a chain of resistors and capacitors coupled to a plurality of about 10-25 discrete electron multiplier disposed structures (dynodes). Collectively, such an arrangement provides a plurality of stages that when operated with voltages between about 1000-5000V enable gains often between about than 105 up to about 107. Beneficially, the dynode (discrete) geometry and operating parameters is often utilized as part of an ion detector when configured to operate with mass analyzers, such as, but not limited to mass filters, ion traps, and time of flight mass spectrometers where significant variation of ion flux is common when operating in various scanning modes or when recording time transients.
The potential difference between a pair of dynodes is often designed so that an electron striking a dynode can produce more than one secondary electron. The average number of secondary electrons per primary electron produced at a particular dynode is the gain of that stage of the electron multiplier with the gain of the entire electron multiplier being the product of the gain at every stage from the cathode to the last dynode. Increasing the voltage applied to the electron multiplier typically increases the voltage between dynodes, increasing the gain of each stage, thereby increasing the gain of the entire multiplier. Operating these detectors with high gain is desired for the detection of low-level signals in order to improve signal-to-noise ratio. However, such high gain values and thus high secondary fluxes result in intense electron currents in the final stages of the current amplification along the various dynode structures. While the use of discrete dynode architecture allows for better control of individual dynodes in the final part of the chain, the beneficial design still cannot in total prevent strong electron currents from hitting dynodes near the output of the device.
One of the primary reasons for aging when utilizing discrete dynode electron multipliers is the carbon deposition on the surface of one or more dynodes which are adjacent the anode of the instrument. The accumulation of excessive carbon deposition has been attributed to the higher doses per unit area of secondary electrons from the dynodes near the anode that enable a carbon to become bonded to the dynode surfaces, which reduces the secondary yield. As part of the phenomenon, the deleterious buildup of carbon occurs more rapidly in poor vacuum conditions, most typical of ion trap instruments. Those of ordinary skill in the art have applied approaches to resolve this issue to include: 1) lowering the background pressure to reduce the carbon buildup; 2) increasing the active surface area of the dynodes under electron impact; and 3) disassembly of the dynodes structures to clean and/or refurbish the device. However, while such approaches have been shown to somewhat ameliorate the aging process of the dynode structures, they are often not always desirable because of the technical challenges and associated costs.
Background information for an electron multiplier that limits the response of the instrument when subjected to a large input signal for an initial period of time, is described and claimed in, U.S. Pat. No. 6,841,936, entitled, “FAST RECOVERY ELECTRON MULTIPLIER,” issued Jan. 11, 2005, to Keller et al., including the following, “[a]n improved electron multiplier bias network that limits the response of the multiplier when the multiplier is faced with very large input signals, but then permits the multiplier to recover quickly following the large input signal. In one aspect, this invention provides an electron multiplier, having a cathode that emits electrons in response to receiving a particle, wherein the particle is one of a charged particle, a neutral particle, or a photon; an ordered chain of dynodes wherein each dynode receives electrons from a preceding dynode and emits a larger number of electrons to be received by the next dynode in the chain, wherein the first dynode of the ordered chain of dynodes receives electrons emitted by the cathode; an anode that collects the electrons emitted by the last dynode of the ordered chain of dynodes; a biasing system that biases each dynode of the ordered chain of dynodes to a specific potential; a set of charge reservoirs, wherein each charge reservoir of the set of charge reservoirs is connected with one of the dynodes of the ordered chain of dynodes; and an isolating element placed between one of the dynodes and its corresponding charge reservoir, where the isolating element is configured to control the response of the electron multiplier when the multiplier receives a large input signal, so as to permit the multiplier to enter into and exit from saturation in a controlled and rapid manner.”
Background information for a photomultiplier detector that includes a gain control circuit to provide feedback to a dynode situated near the anode, is described and claimed in, U.S. Pat. No. 5,367,222, entitled, “REMOTE GAIN CONTROL CIRCUIT FOR PHOTOMULTIPLIER TUBES,” issued Nov. 22, 1994, to David M. Binkley, including the following, “[a] gain control circuit (10) for remotely controlling the gain of a photomultiplier tube (PMT (12)). The remote gain control circuit (10) may be used with a PMT (12) having any selected number of dynodes (DY). The remote gain control circuit (10) is connected to the last dynode nearest the anode (16) in the dynode string which controls the total dynode supply voltage and influences the gain of each dynode (DY). The remote gain control circuit (10) of the present invention includes an integrated-circuit operational amplifier (U1), a high-voltage transistor (Q1), a plurality of resistors (R), a plurality of capacitors (C), and a plurality of diodes (D). Negative feedback is used to set the last dynode voltage proportional to a voltage controlled by the gain control voltage delivered by a voltage source such as a digital-to-analog converter. The control circuit (10) of the present invention is connected to the last dynode using a single connecting wire (22).”
Background information for a photomultiplier detector having gain control through change of the bias on at least one of the dynodes, is described and claimed in, U.S. Pat. No. 4,804,891, entitled, “PHOTOMULTIPLIER TUBE WITH GAIN CONTROL,” issued Feb. 14, 1989, to Harold E. Sweeney, including the following, “[i]mproved gain control in a photomultiplier tube having a plurality of dynode stages is achieved through manual or automatic change of the bias voltage on at least one of the several dynodes between the anode and cathode of the tube. By such means, maximum tube gain change is obtained with a minimum of bias voltage swing.”
Background information for a photomultiplier detector having automatic gain control, is described and claimed in, U.S. Pat. No. 3,614,646, entitled, “PHOTOMULTIPLIER TUBE AGC USING PHOTOEMITTER-SENSOR FRO DYNODE BIASING,” issued Oct. 19, 1971, to Earl T. Hansen, including the following, “[a] photomultiplier tube automatic gain control circuit wherein the biasing potentials between a plurality of adjacent dynodes are varied inversely as the amplitude of the photomultiplier output signal. The output signal is detected and applied to a photoemitter-sensor connected in shunt with the biasing network for the aforesaid dynodes.”
Accordingly, there is a need in the field of particle detection to improve the operational lifetime for such structures when operated at high gains. The present invention addresses this need, as disclosed herein, by providing a novel intermediate dynode structure and coupled circuit to regulate the gain and thus the intensity of the secondary emission of one or more downstream dynodes near the output of the device no matter high strong of an input signal.